The present invention relates to a chat system for performing the communication real time using a character or the like, a chat system terminal unit, a key management server for supplying a key to the encryption of the communication, and a recording medium readable between computers on a computer network such as the internet or the intranet.
FIG. 1 is a diagram for explaining the principle of a conventional chat system. In this chat system, communication channels A to C (51 to 53) are opened on a chat server 50 connected to a computer network. The user connects the chat server 50 by chat clients (terminal units) 60 to 69 and thus selectively connects the channels A to C (51 to 53). As a result, the communication using a character or the like can be accomplished between users connected to the same channel. Generally, the information transmitted from a user is distributed to all the users connected to the same channel. Also, even on a channel connected with three or more users, one-to-one communication between users is possible.
FIG. 2 is a block diagram showing a configuration example of the essential parts of the conventional chat system. In this chat system, first, an input unit 14 of a chat client 5 receives an input signal from a keyboard (not shown). In FIG. 2, only a single chat client 5 is shown as a representative. An input signal received by the input unit 14 is transmitted by a transmission section 16 to a channel X(4) which, in the chat server 2 is selected by a channel selection section 10. A signal arriving from other chat clients through the channel X(4) in the chat server 2 is received by a receiving section 17. The signal received by the receiving section 17 is output by an output unit 19 to and is displayed on a display screen (not shown).
In this chat system, the communication between the chat client and the chat server is carried out in a plaintext and therefore a communication text is liable to be eavesdropped midway of the communication line.
A method suggested for preventing the eavesdropping is to encrypt the communication text using a secret key prepared by a chat server. FIG. 3 is a block diagram showing an example configuration of the essential parts of a conventional chat system with the communication text encrypted. The chat server 7 of the chat system includes a channel secret key generation section 34, an encryption section 33, a channel secret key request receiving section 31 and a channel secret key distribution section 32. The channel secret key generation section 34 generates a channel secret key unique to each channel for encrypting/decrypting the communication data exchanged through a channel in the server 7 managed by the server 7. The encryption section 33 encrypts the channel secret key generated by the channel secret key generation section 34. The channel secret key request receiving section 31 receives a distribution request of the channel secret key unique to the channel X(4) from a chat client 6 through the channel X(4). When the channel secret key request receiving section 31 receives the distribution request, the channel secret key distribution section 32 distributes the channel secret key unique to the channel X(4) encrypted by the encryption section 33 to the chat client 6 through the channel X(4). In FIG. 3, only one chat client 6 is shown to represent all chat clients.
The chat client 6 of the chat system includes a channel selection section 10 for selecting a connected channel, a channel secret key request section 11 for requesting a channel secret key unique to the channel X(4) through the channel X(4) selected by the channel selection section 10, a channel secret key receiving section 12 for receiving the channel secret key requested by the channel secret key request section 11, a channel secret key decryption section 13 for decrypting the channel secret key received by the channel secret key receiving section 12, and an input unit 14 for receiving the input signal from a keyboard (not shown).
The client 6 of the chat system includes an encryption section 15 for encrypting the input signal received by the input unit 14 using the channel secret key decrypted by the channel secret key decryption section 13, a transmission section 16 for transmitting the input signal encrypted by the encryption section 15 to the channel X(4), a receiving section 17 for receiving the signal arriving from other channel clients through the channel X(4), a decryption section 18 for decrypting the signal received by the receiving section 17 using the channel secret key, and an output unit 19 for outputting and displaying the received signal decrypted by the decryption section 18 in the form of character on a display screen (not shown).
With the chat system having this configuration, the chat server 7 generates a secret key of each channel at the channel secret key generation section 34 and encrypts the channel secret key at the encryption section 33.
The chat client 6 selects the connected channel at the channel selector 10, and the channel secret key request section 11 requests the chat server 7 for a channel secret key unique to the channel X(4) through the channel X(4) selected by the channel secret key request section 11. The chat server 7 accepts the request at the channel secret key request receiving section 31 and distributes the encrypted channel secret key unique to the channel X(4) to the chat client 6 through the channel X(4).
The chat client 6 receives the requested channel secret key unique to the channel X(4) at the channel secret key receiving section 12, and decrypts the received channel secret key at the channel secret key decryption section 13. The chat client 6 encrypts/decrypts the signal transmitted/received to and from the channel X(4) at the encryption section 15/decryption section 18 using the decrypted channel secret key.
In the above-mentioned chat system, however, the communication text on the chat server 7 can be modified into a plaintext and eavesdropped by reconstructing the chat server 7. Even when the plaintext is encrypted by the secret key prepared by the chat server, therefore, the likelihood of eavesdropping is not eliminated.